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    <title>Document</title>
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    <script>
        function fun(n, o) {
            console.log(o); // undefined
            return {
                fun: function (m) {
                    return fun(m, n);
                }
            };
        }
        var a = fun(0); // 0
        a.fun(1);   // 
        a.fun(2);   //
        a.fun(3);   //

        var b = fun(0).fun(1).fun(2).fun(3);

        var c = fun(0).fun(1);
        c.fun(2);
        c.fun(3);
        //问:三行a,b,c的输出分别是什么？
        console.log(a);
        console.log(b);
        console.log(c);
    </script>
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